GaN-Based Isolated DC-DC Converter for Space Application

Technology absorption in the space and defense radiation-hardened electronics sector progresses at a slower pace than in the commercial and industrial world. Thus, power converters used in some space and defense applications are typically bulkier, less efficient, and more expensive than their commercial or industrial counterparts. The main drivers for the reduction in performance and increased cost of radiation-hardened power converters come from the increase in physical size and the additional qualification testing that is necessary to produce silicon (Si) devices and power metal-oxide semiconductor field-effect transistors (MOSFETS) that can operate reliably in high radiation environments. Besides offering better performance and power density than Si MOSFETS, radiation testing results show that gallium-nitride (GaN) FETs are inherently more robust than Si MOSFETs when exposed to high doses of radiation. This makes GaN FETS an attractive replacement for bulkier and generally more expensive power MOSFETs for space radiation-hardened applications.

The design of an isolated dc-dc zero voltage switching (ZVS) full-bridge converter using a current doubler synchronous rectifier on the secondary side to achieve high efficiency and high power density while abiding to circuit design constraints encountered in rad-hard applications is described in this paper. The prototype uses a combination of radiation-hardened parts and industrial-rated components with radiation-hardened equivalent packages to keep prototyping costs down and parts procurement under control. Fig. 1 shows a high-level block diagram for converter topology. The power stage uses hermetically packaged rad-hard GaN FETs from Freebird Semiconductor. Low-profile planar magnetics using discrete helical-windings arrangements were selected for the transformer, and required resonant and output inductors for the current doubler rectifier.

Hard-switched and ZVS phase-shifted modulation schemes are compared in terms of component stress, efficiency, and conducted electromagnetic interference. Design methodology for hard-switched and ZVS full-bridge converters, and magnetics design tradeoffs are presented. Also, the main challenges encountered in designing radiation hardened power converters and their effects on converter design choices are briefly discussed.